Size effect on magnetism and large magnetocaloric effect of Haldane chain antiferromagnet Er2BaNiO5

Abstract

We investigated the size effect on magnetocaloric effect (MCE) of Haldane spin-chain antiferromagnet Er2BaNiO5. The bulk sample was prepared by solid state route and the nano-size samples (N-4 h, N-8 h) were produced by using the high-energy planetary ball mill machine. The crystal structure was obtained by the refinement of XRD results and the SEM images. The present magnetization results indicated that the decreasing of average crystallite size induced enhancement of magnetization and paramagnetic-like behavior. The zero-approaching of θp values for N-8 h revealed the suppression of the dominant antiferromagnetic (AFM) interactions in bulk sample. The fitting results of Haldane gap stated clearly its dramatical reduction which is consistent with the weaken of magnetic interactions. The size effect also presented great influence on MCE. The negative MCE in bulk sample disappeared and instead of monotonically increasing of MCE in N-4 h and N-8 h samples. The maximum entropy change (-ΔSmax) of N-8 h increased nearly 3.2 times compared to bulk sample. The increasing of PM ions in nano-samples were verified by the ESR results, in which the bulk sample showed unavailable ESR signals due to the complex competing 3D AFM interactions. While for the nano-samples, the ESR signals of Er3+ and Ni2+ ions were observed with the g value of gEr = 6.1 and gNi = 2.28. The size-effect on MCE in Er2BaNiO5 opened new approach of the research on magnetic refrigeration.